Coal feeders of the type mentioned above are sold by the Stock Equipment Co. in Cleveland, Ohio. The conveyor apparatus comprises a conveyor belt with load measuring means permitting automatic control of the coal mill to be supplied. Frequently, the coal mill and, consequently, its supply ducts are subjected to superatmospheric pressure, and this excess pressure will be present also in the conveyor apparatus and at its inlet where coal is charged from the bunker via the feed pipe. The coal column which will exist in the feed pipe acts as a pressure seal or a plug and so minimizes the danger that back ignition from the coal burners through the conveyor apparatus will reach the bunker where the coal is stored. Servomechanism means are provided to control an outlet valve at the bunker outlet in order to stabilize the height of the coal column in the feed pipe, the conveyor apparatus at the outlet of the feed pipe in turn being controlled by a servomechanism responsive to the coal demand of the coal mill or of the coal burners.
In the past, the feed pipe was a simple cylindrical tube provided with detectors for column height control. If the coal to be combusted is somewhat sticky, its natural slope angle may even become negative and a "bridge" will form in the feed pipe; this phenomenon is sometimes designated "arching." Arching commences by sticking of the bulk material at the inner walls of the feed pipe and the thickness of this deposit will increase with the distance from the pipe outlet. Finally, the pipe section will be completely blocked, and coal can no longer fall under gravity onto the conveyor apparatus. Above the bridge, the coal column will grow rapidly, the column height monitors will signal maximum height, and the bunker outlet valve will close. Under the circumstances, the servo system will shut off the entire plant.
Once a feed pipe has been blocked in the manner described, service personnel will try to jar or vibrate the feed pipe by means of hammer shocks in an attempt to make the arch collapse. It is evidence that this is a very unsafe and uneconomical solution.